The present invention relates to an apparatus and a method for making a single crystalline nitride substrate; and, more particularly, to an apparatus and a method for preventing cracks from being generated in a single crystalline nitride substrate.
A GaN single crystalline substrate, as an example of a single crystalline nitride substrate, will be described. Generally, the GaN materials has a melting point more than of 2400 and the dissociation pressure of nitride in the GaN materials is about ten thousand atmospheres. Accordingly, this high melting point and high dissociation pressure make it impossible to create a large single crystalline GaN bulk using typical growing methods of the semiconductor crystals. A needle-shaped crystal growing method, in which a gallium gas directly reacts on an ammonia gas at a high temperature of about 1000xc2x0 C. to 1150xc2x0 C., and a plate-shaped crystal growing method, in which nitrogen is dissolved in liquid gallium at a high temperature of about 1500xc2x0 C. to 1600xc2x0 C. and at a high nitrogen pressure corresponding to about 20000 atmospheres has been used to create a single crystalline GaN bulk (hereinafter, referred to as a GaN bulk).
However, these crystal growth methods have made a small-sized GaN bulk which has only a few millimeters in size and about 100xcexcm in thickness. Accordingly, it is impossible to achieve a commercial success in using the GaN bulk.
To solve the above problem, a hydride vapor phase epitaxy has been used to create the GaN bulk at a growing rate of 100xcexcm/hour. That is, after forming a thick GaN film on a parent substrate, such as sapphire or SiC substrate, the parent substrate is removed and then the GaN bulk is finally formed.
The removal of the parent substrate is carried out by the mechanical polishing method or laser beam. In particular to laser, as shown in FIG. 1, after forming the thick GaN film on the parent substrate at a high temperature of about 1000xc2x0 C. to 1100xc2x0 C., the thick GaN film on the parent substrate is cooled down to a room temperature. After increasing the temperature of the parent substrate up to about 600xc2x0 C., the thick GaN film is separated from the parent substrate using laser beam in an additional apparatus different from the hydride vapor phase epitaxy (xe2x80x9cLarge free-standing GaN substrate by hydride vapor phase epitaxy and laser induced lift-off,xe2x80x9d by K. Kelly et al, Jpn. J. Appl. Phys. Vol. 38, No. 3A (pt 2), 1999).
In the above-mentioned hydride vapor phase epitaxy, since the thick GaN film is formed on the sapphire substrate at a high temperature and it is cooled down to the room temperature, cracks are generated by the lattice mismatch and thermal expansion coefficients between the GaN film and the sapphire substrate. Because of these cracks, the GaN bulk is restricted within a small-sized substrate and electric characteristics therein are also deteriorated.
It is, therefore, an object of the present invention to provide an apparatus and a method for preventing cracks from being generated in a single crystalline nitride substrate which is made by a hydride vapor phase epitaxy method.
Another object of the present invention is to provide an apparatus and a method for forming a large single crystalline nitride substrate on a commercial basis.
In accordance with an aspect of the present invention, there is provided an apparatus for forming a compound semiconductor substrate, the apparatus comprising: a reacting chamber for forming a single crystalline film on a parent substrate; a heating chamber connected to the reacting chamber within a processing channel, wherein the single crystalline film is separated from the parent substrate at a higher temperature than a room temperature; and a supporter for supporting the single crystalline film and the parent substrate and maintaining the single crystalline film in a predetermined temperature.
In accordance with another aspect of the present invention, there is provided a method for forming a compound semiconductor substrate, the method comprising the steps of: a) preparing a parent substrate; b) forming a single crystalline film on the parent substrate in a reacting chamber; c) maintaining the single crystalline film in a predetermined temperature which is higher than a room temperature; and d) illuminating laser beam on a backside of the parent substrate and separating the single crystalline film from the parent substrate.
According to the present invention, a thick GaN film is formed on a parent substrate, such as sapphire (Al203), spinel (MgAl2O4) or silicon carbide (SiC), which has the lattice mismatch with the single crystalline GaN film and a different thermal expansion coefficient, and the parent substrate is heated up to a range of 600xc2x0 C. to 1000xc2x0 C. In this temperature range, the single crystalline GaN film is separated from the parent substrate by laser beam.